Heat shrinkable polymer films have gained substantial acceptance for such uses as the packaging of meats and other articles of food. This description will detail the usage of films for packaging meat; it being understood that these films are also suitable for packaging other products such as frozen foods and cheeses. Some of the films embodying this invention are normally used as heat shrinkable bags supplied to the meat packer with one open end, to be closed and sealed after insertion of the meat. After the product is inserted, air is normally evacuated, the open end of the bag is closed, such as by heat sealing, or applying a metal clip, and finally heat is applied, such as by hot water, to initiate film shrinkage about the meat.
In subsequent processing of the meat, the bag may be opened and the meat removed for further cutting of the meat into user cuts, for retail sale, for example, or for institutional use.
Successful shrink bags must satisfy a multiplicity of requirements imposed by both the bag producer and the bag user. Of primary importance to the bag user is the capability of the bag to survive physically intact the process of being filled, evacuated, sealed closed, and heat shrunk. The bag must also be strong enough to survive the material handling involved in moving the contained meat, which may weigh 100 pounds or more, along the distribution system to the next processor, or to the user. Frequently, the meat will have protruding bones that will puncture shrink bags that are not sufficiently strong. Thus, the bag must be strong enough to physically protect the meat.
It is also highly desirable to the bag user that the bag serve as a barrier to infusion of gaseous material from the surrounding environment. Of particular importance is provision of an effective barrier to infusion of oxygen, since oxygen is well known to cause spoilage of meat.
The bag producer requires a product which can be produced competitively while meeting the performance requirements of the user. Thus, the bag material should be readily extrudable, and susceptible to cross-linking by irradiation, with sufficient leeway in process parameters as to allow for efficient film production. The process should also be susceptible to efficient extended production operations. In the irradiation process, the film produced must have improved strength while the inner layer maintains sufficient sealability.
It is important to users of shrink bags that once the meat or other food article is placed in the bag, the bag can be heat sealed to form an air tight bond. Generally, the heat seal is formed by applying heat and pressure to the opposing sides of the bag mouth until the opposing inner layers of the film have fused together.
One of the problems encountered when heat sealing bags made from multiple layer films is that the sealing process causes the film to become deformed in the area where the heat is applied. A solution to this problem known in the art has been to cross-link the film layers by irradiation prior to heat sealing. Cross-linking the film provides improved toughness and increases the heat seal temperature range.
However, cross-linked thermoplastic films are more difficult to melt and produce weaker seals than unirradiated films when heat sealed. Users require that the seals maintain their integrity when the bag containing meat or other food article is immersed in hot water to shrink the film. A bag with weak heat seals that rupture when the bag is shrunk, is of no use. Thus, there is a need for an irradiated multiple layer film which can be made into a bag that will have strong seals when heat sealed.
It is known that heat shrinkable bags for food packaging may be made from multiple layer films in which individual layers have different degrees of cross-linking. Such multiple layer films have been fabricated by forming and irradiating the layers individually and then laminating or extrusion coating the layers to form the multiple layer film. These multiple step fabrication methods produce a more costly film.
Canadian Patent, 1,125,229 discloses a film structure having a heat sealable inner layer and an outer layer wherein the outer layer is cross-linked to a larger extent then the heat sealable layer. The differential cross-linking is achieved by adding a cross-linking enhancer compound to the outer layer, forming the structure, and then irradiating. The irradiation enhancer allows the irradiation dosage to be lowered to a point where the heat sealable inner layer is not adversely affected in its heat sealing characteristics by the radiation. However, the lower irradiation dosage does not produce a bag with the strength and toughness required by users.
U.S. Pat. No. 4,724,176 to Sun discloses a heat shrinkable container having an unirradiated inner layer, a barrier layer and an irradiated outer layer. This film is coextruded and the outer layer is irradiated by precisely controlling the depth of material that the electron beam irradiation penetrates. This invention limits the cross-linking to the outer layer and therefore, does not improve the strength of the inner layer by cross-linking.
U.S. Pat. No. 5,055,328 to Evert discloses a multiple layer film in which the inner layer contains an antioxidant cross-linking inhibitor to control the degree of cross-linking.
U.S. Pat. No. 4,894,107 to Tse discloses oriented or unoriented multiple layer films with barrier layers that are irradiated for cross-linking. This invention does not teach the selection of polymers for individual layers with different degrees of cross-linking when irradiated.
Many of the multilayer heat shrinkable films with an uncross-linked inner layer and a cross-linked outer layer previously known in the art were made by extruding the outer layer separately from the inner layer. After the outer layer was extruded and irradiated, it would be laminated with the inner layer and any other additional layers to form the multilayer film. In the present invention, the additional expense of separately extruding the irradiated and unirradiated layers and then laminating the layers together is avoided. All of the layers of the films in the present invention can be coextruded and the entire film structure can be exposed to EB radiation. Such films would be widely accepted by those skilled in the art and meet with substantial commercial success.
It is an object of this invention to provide multiple layer cross-linked films with improved sealability and toughness. It is a further object of this invention to provide a coextruded multiple layer cross-linked film having these improvements after subjecting the fabricated multiple layer film structure to a single dose of irradiation. Another object of this invention is to provide a meat or food article packaging bag that will maintain the integrity of heat seals when it is shrunk wrapped.
It should be understood that the objectives stated in or inferred from this specification do not limit the invention as defined in the claims.
Irradiation of polymers causes the formation of covalent bonds between different polymer chains. This process is called "cross-linking". The overall effect of cross-linking is that the molecular weight of the polymer steadily increases with dose, leading to branched chains, until ultimately a tri-dimensional network is formed. This three dimensional network is referred to as the "gel fraction". The gel fractions disclosed herein have been determined in accordance with ASTM D2765. The gel fraction molecules are insoluble while the unlinked molecules remain soluble and are referred to as the "sol fraction". These molecules are separated from the network although they may be highly branched and can be extracted from the bulk polymer by a process that uses a proper solvent. Thus, the gel fraction can be easily measured to determine the extent of cross-linking for various radiation doses.
It has been known in the art that irradiation of polymeric multiple layer films cross-links the layers and produces a film with improved toughness and strength. However, cross-linking also raises the normal melting temperature of a polymer and consequently reduces the heat sealability. Surprisingly, it has been found in the practice of this invention that by the selection of different polymers for the various layers of a multiple layer film, it is possible to have extensive cross-linking in one layer and a minimum amount of cross-linking in another layer when the film is irradiated. This allows the outer layer of a multiple layer film to be cross-linked to provide increased strength and toughness while the inner layer is not cross-linked and retains its heat sealability characteristics.